1. Field of Invention
The invention relates to the field of signal processing technology, and particularly to the weighting and combining of signals used in antenna arrays and other communication systems.
2. Description of Related Art
In communication receiving stations, for example mobile cellular installations, systems equipped with multiple-antenna arrays offer improved receiver performance. The signals arriving on the antenna channels are sampled and then weighted and combined to improve performance. Those antenna arrays, for instance as discussed in the U.S. Patent Application entitled "ADAPTIVE COMMUNICATION SYSTEM AND METHOD USING UNEQUAL WEIGHTING OF INTERFERENCE AND NOISE", filed May 1, 1997, and assigned to the same assignees as this application, incorporated here by reference, can among other things reduce the effects of multipath fading and interference by deriving weights for each antenna channel that lead to improved receiver performance. In such systems it is necessary to accurately sample the incoming data stream to develop the parameters, such as estimated noise and interference, needed to generate high-performance weights. The weights developed from a window of K past samples, where illustratively, K=14, described in the aforementioned U.S. Patent Application are applied to the next-arriving sample in the data stream, as illustrated in FIG. 4 herein. Once the symbol corresponding to that sample is detected, the window used to generate weights moves one time unit to the right, and the newly generated weights (using 13 prior window symbols plus the just-detected symbol)are applied to the subsequent symbol, in sliding fashion.
This approach of applying currently generated weights to the next-arriving symbol has the advantage of straightforward implementation, but falls short of ideal for at least two reasons. One is that the channel is varying over the K-symbol window used to sample the channel. In terms of channel variation, as illustrated herein in FIGS. 4 and 5 when incoming data streams are sampled, in the typical approach to weight application as outlined in the forementioned U.S. Patent Application, the weights derived from the (14-symbol) window at time t are applied to the next-arriving signal sample, at time (t+1).
However, the weight derived from the window is typically most representative of the instantaneous condition of the channel, not at the end, but in the middle of the sample window. By the time the next-arriving data symbol appears, the characteristics of the channel can vary significantly, particularly in fast-fading environments such as mobile radio. Applying weights which most accurately represent the channel several symbols ago to the next-resolved symbol can therefore degrade performance.
Moreover, in terms of encoding technique, data transmitted in the industry standard IS-136 format is always transmitted in differentially encoded (phase shift) form. On the detection side, either coherent or differential detection can be used, as understood by persons skilled in the art. For instance, current commercial base station equipment typically detects data arriving from an antenna or antenna array by differential detection, that is, detecting phase shifts to decode data. Differential detection typically requires 1 dB greater signal-to-noise ratio (SNR) than absolute phase (coherent) techniques with an ideal phase reference.
However, the coherently-sliced reference signals used to assess the channel and generate initial weights are themselves detected coherently, in the weight-adaptive technique described in the foregoing patent application. In that approach the channel weights may be generated for example by an outboard signal processing applique which determines channel characteristics coherently, then passes weighted and combined signals to an existing base station unit which performs data detection differentially.
Besides channel variation, samples taken at one stage of a receiver system therefore have different noise, interference, and other characteristics which when introduced into another and different type of processing in the system, cause added errors and signal degradation. In conventional approaches to the sampling of data streams to generate channel weights, neither channel variation nor the fact that various stages of data processing may be carried out differently, are taken into account.